1. Field of the Invention
The present disclosure relates to a liquid discharging recording head that discharges liquid.
2. Description of the Related Art
In a liquid discharging recording apparatus that performs recording by discharging liquid such as ink, a liquid discharging recording head is mounted. The liquid discharging recording head includes a recording element substrate (element substrate) having discharge ports. The liquid discharging recording apparatus performs image recording by discharging liquid supplied to the recording element substrate from the discharge ports so that the liquid is attached onto a recording medium.
In recent years, recording speed has been increased by increasing the length and number of discharge port arrays provided in one recording element substrate or arranging a plurality of recording element substrates in a discharge port array direction on a liquid discharge recording head.
Such a liquid discharge recording head is disclosed in Japanese Patent Laid-Open No. 2009-101578. FIG. 5A illustrates a full-line liquid discharging recording head in which recording element substrates H1100 (H1100a to H1100f), each having a plurality of discharge port arrays H1106, are arranged in a staggered manner in a discharge port array direction on a support plate H1200 to form discharge port arrays corresponding to the width of a recording medium.
FIG. 5B is a partial cross-sectional view of the liquid discharging recording head, taken along line VB-VB of FIG. 5A. Liquid supplied from an un-illustrated liquid storage tank flows through liquid supply passages H1201a to H1201d provided in the support plate H1200, and are then supplied to liquid supply ports H1101a to H1101d of the corresponding recording element substrates H1100.
When a pulse signal is transmitted from a liquid discharging recording apparatus body to heaters (recording elements) on the recording element substrates H1100, heat energy is applied to discharge the liquid from the discharge ports, whereby image recording is performed. Heat generated in the recording element substrates H1100 during recording is radiated outside via the support plate H1200.
In the liquid discharging recording head described in Japanese Patent Laid-Open No. 2009-101578, the temperature tends to become lower in an outer peripheral portion of each recording element substrate H1100 than in a center portion, and the in-plane temperature distribution of the recording element substrate H1100 is non-uniform.
The reason why such a phenomenon occurs can be explained by the difference of the heat transfer passage between the center portion and the outer peripheral portion of the recording element substrate H1100.
First, heat transfer in the center portion will be described. For example, heat generated by the heaters provided in the recording element substrate H1100 is diffused between the liquid supply ports H1101a and H1101b, and is further transferred to a portion of the support plate H1200 between the liquid supply ports H1201a and H1201b (FIG. 5B). While the liquid supply passages H1201 are in contact with the liquid, the thermal conductivity of a liquid is much lower than that of a solid, and therefore, heat is rarely transferred to the liquid. The heat is transferred in the discharge port array direction in the support plate H1200 and is finally diffused out of the support plate H1200 through a long transfer passage.
In contrast, in the outer peripheral portion, heat is diffused from ends of the recording element substrate H1100 to an outer side of the support plate H1200 without being blocked by the liquid supply passages H1201. For this reason, the temperature becomes lower in the outer peripheral portion than in the center portion of the recording element substrate H1100, and this causes unevenness in temperature in the plane of the recording element substrate H1100.
In general, the amount of discharged liquid increases as the temperature of the recording element substrate increases. Hence, if the unevenness in temperature occurs in the plane of the recording element substrate, the amount of discharged liquid also becomes uneven. This causes unevenness in density of a recorded image, and deteriorates image quality. Particularly when input heat energy per unit time is increased for high-speed image recording, the unevenness in temperature in the plane of the recording element substrate increases further.
In contrast, in a liquid discharging recording head described in Japanese Patent Laid-Open No. 2008-194940, resistors are provided around portions on a support plate where recording element substrates are mounted. The temperature of each recording element substrate is controlled by applying a signal from a liquid discharging recording apparatus to the corresponding resistor so as to appropriately generate heat in the resistor while monitoring a temperature sensor provided on the recording element substrate. Since the liquid discharging recording head of Japanese Patent Laid-Open No. 2008-194940 adopts this structure, an outer peripheral portion of the recording element substrate can be heated.
However, since the in-plane temperature distribution of each recording element substrate is not uniform in a direction intersecting a direction of discharge port arrays of the recording element substrate, even if the outer peripheral portion of the recording element substrate is uniformly heated, the in-plane unevenness in temperature of the recording element substrate is not reduced.
Specifically, the fall in temperature from the temperature in the center portion is larger at both ends (outer peripheral portion) of the recording element substrate in a direction of the discharge port arrays (a direction parallel to the discharge port arrays) than at both ends of the recording element substrate in the direction intersecting the discharge port arrays (a direction perpendicular to the discharge port arrays). For this reason, if the temperature distribution in the direction of the discharge port arrays is made uniform, the temperature excessively increases at both ends in the direction intersecting the discharge port arrays, and as a result, the in-plane unevenness in temperature of the recording element substrate is not reduced.
Such a difference in temperature distribution in both the directions is considered to be caused by the difference in distance from the outermost recording element to the end of the recording element substrate between the directions and the influence of arrangement of the recording element. However, it is difficult to adjust the distance and arrangement, because of restrictions on a circuit of the recording element substrate.